ISTANBUL - Turkish police fired tear gas, water cannon and rubber pellets on Thursday to try to stop thousands of people, some armed with fire bombs and fireworks, from defying a ban on May Day rallies and reaching Istanbul's central Taksim square.

Citing security fears, authorities shut parts of the city's public transport system and deployed thousands of riot police, blocking access to Taksim, a traditional union rallying point and the focus of weeks of anti-government protests last summer.

Prime Minister Tayyip Erdogan, who warned last week against efforts to march on Taksim, has cast both last year's street protests and a corruption scandal dogging his government since December as part of a plot to undermine him.

The Istanbul governor's office said it had advance information that "illegal terror organizations and their extensions" would resort to violence to stoke unrest.

Pockets of protesters played cat-and-mouse in several neighborhoods on the fringes of Taksim, a huge square surrounded by stores, restaurants and hotels usually thronged by commuters, shoppers and tourists.